The invention relates generally to monolithic devices and more particularly to a method of monolithically interconnecting electrical devices such as photovoltaic (PV) cells on substrates including flexible substrates.
PV cells need to be electrically connected via an interconnection process in order for individual PV cells to be fabricated into modules. The conventional interconnection approach involves connecting discrete cells together via shingling or metallic ribbons. In the conventional approach, interconnected cells do not maintain a common substrate.
Monolithic integration refers to the process of interconnecting cells as part of the cell fabrication process. Monolithic integration typically is implemented for thin film PV modules where PV layers are deposited over large area substrates. Scribe and pattern steps are often used to divide the large area into electrically interconnected cells while maintaining a common substrate. This approach is typically applied to solar cells that are deposited on glass.
Several approaches exist for implementing monolithic integration; and each approach has various advantages and disadvantages related to the fabrication sequence, required tools, and material interactions, among other things.
One of the greatest challenges in thin film PV fabrication relates to the need to isolate the top contacts of neighboring cells, i.e., scribe through the top conducting outer layer without damaging the underlying layers. Three scribes are typically necessary to form a monolithic interconnect. The spacing between scribes should be wide enough to overcome the possibility of unwanted electrical connections. The total area occupied by the scribes plus any space between scribes should ideally be as small as possible to maximize the absorbing area of the PV cell. Mechanical scribing is not practical for flexible substrates; and laser scribing is challenging if the underlying layers are more highly absorbing than the overlying layer (in the case of CIS cells, the transparent conductive oxide (TCO) layer).
Thin film PV modules are implemented by dividing the module into individual cells that are series connected to provide a high voltage output. A conventional monolithic PV cell interconnect process employs a sequence of steps such as shown in FIG. 3. The process begins by depositing a first conducting layer 60 on a substrate 62. The first conducting layer 60 is scribed using a linear cut 64 across the module. A semiconductor layer 66 such as a CIGS layer is then deposited as depicted in FIG. 3. A second scribe 68 parallel to the first scribe 64 isolates the CIGS layer 66 into individual PV cells. A second conducting layer 70 that may be a transparent conductive oxide (TCO) is then deposited as also depicted in FIG. 3. The process is completed with a third scribe 72 to form the series connection 74 in which the TCO from the second conducting layer 70 connects the top of one cell 76 to the bottom of the next cell 78.
In view of the above, it would be both advantageous and beneficial to provide a method of monolithically interconnecting electrical devices such as PV cells on a common flexible substrate while avoiding the challenges associated with known monolithic integration techniques.